FIX Fix feature detection when `obj.constructor` is undefined (#2520)

Currently the following code fails:
```py
from js import eval
eval("Object.create(null)")
```
with:
```py
Traceback (most recent call last):
  File "<console>", line 1, in <module>
JsException: TypeError: Cannot read properties of undefined (reading 'name')
```
This fixes it.

Makefile

@@ -210,6 +210,7 @@ dist/test.tar: $(CPYTHONLIB) node_modules/.installed
 	for testname in $(TEST_EXTENSIONS); do \
 		cd $(CPYTHONBUILD) && \
 		emcc Modules/$${testname%.*}.o -o $$testname $(SIDE_MODULE_LDFLAGS) && \
+		rm -f $(CPYTHONLIB)/$$testname && \
 		ln -s $(CPYTHONBUILD)/$$testname $(CPYTHONLIB)/$$testname ; \
 	done
 

docs/project/changelog.md

@@ -61,6 +61,9 @@ substitutions:
   translated to negative Python ints.
   {pr}`2484`
 
+- {{ Fix }} Pyodide now correctly handles JavaScript objects with `null` constructor.
+  {pr}`2520`
+
 - {{ Fix }} Fix garbage collection of `once_callable` {pr}`2401`
 
 - {{ Enhancement }} `run_in_pyodide` now has support for pytest assertion

src/core/error_handling.ts

@@ -194,6 +194,9 @@ function convertCppException(ptr: number): CppException {
 Tests.convertCppException = convertCppException;
 
 function isPyodideFrame(frame: ErrorStackParser.StackFrame): boolean {
+  if (!frame) {
+    return false;
+  }
   const fileName = frame.fileName || "";
   if (fileName.includes("wasm-function")) {
     return true;

src/core/hiwire.c

@@ -712,7 +712,7 @@ EM_JS_REF(JsRef, hiwire_to_string, (JsRef idobj), {
   return Hiwire.new_value(Hiwire.get_value(idobj).toString());
 });
 
-EM_JS_REF(JsRef, hiwire_typeof, (JsRef idobj), {
+EM_JS(JsRef, hiwire_typeof, (JsRef idobj), {
   return Hiwire.new_value(typeof Hiwire.get_value(idobj));
 });
 
@@ -781,7 +781,7 @@ EM_JS_REF(JsRef, JsObject_Values, (JsRef idobj), {
 EM_JS(bool, hiwire_is_typedarray, (JsRef idobj), {
   let jsobj = Hiwire.get_value(idobj);
   // clang-format off
-  return ArrayBuffer.isView(jsobj) || jsobj.constructor.name === "ArrayBuffer";
+  return ArrayBuffer.isView(jsobj) || (jsobj.constructor && jsobj.constructor.name === "ArrayBuffer");
   // clang-format on
 });
 

src/core/jsproxy.c

@@ -119,6 +119,12 @@ static PyObject*
 JsProxy_Repr(PyObject* self)
 {
   JsRef idrepr = hiwire_to_string(JsProxy_REF(self));
+  if (idrepr == NULL) {
+    PyErr_Format(PyExc_TypeError,
+                 "Pyodide cannot generate a repr for this Javascript object "
+                 "because it has no 'toString' method");
+    return NULL;
+  }
   PyObject* pyrepr = js2python(idrepr);
   hiwire_decref(idrepr);
   return pyrepr;

src/tests/test_typeconversions.py

@@ -1425,6 +1425,17 @@ def test_buffer_format_string(selenium):
         assert array_name == expected_array_name
 
 
+@run_in_pyodide
+def test_object_with_null_constructor():
+    from unittest import TestCase
+
+    from js import eval as run_js
+
+    o = run_js("Object.create(null)")
+    with TestCase().assertRaises(TypeError):
+        repr(o)
+
+
 def test_dict_converter_cache(selenium):
     selenium.run_js(
         """